So Twitter is officially announcing that Buy button today — in a blog post, the company says it will be visible to “a small percentage of U.S. users (that will grow over time).” As a result, users will actually be able to make purchases directly within tweets. If you see something you want, you hit “buy”, bring up a little information, enter your payment and shipping information while still Twitter (it’s encrypted and stored for future purchases), and complete the transaction “in just a few taps.”

And even though the test is starting out on mobile, a company spokesperson said it will be moving onto desktop soon as well.

Twitter says it’s partnering with a number of companies to make the Buy Now button happen, starting social shopping company Fancy, digital content seller Gumroad, fan commerce company Musictoday, and payments company Stripe. The initial sellers include musicians (Demi Lovato, Eminem, Pharrell), nonprofits (Donors Choose, Glide, RED), and retailers (Burberry, The Home Depot).

Gumroad founder and CEO Sahil Lavingia told me his team has been talking to Twitter about possible commerce features for more than a year: “It was more like a when, not if, question for them.” Where Gumroad is involved in these initial purchases, Lavingia said it’s usually working with musicians (though he wants to go broader), with Stripe handling the actual payment processing. When you make the purchase, the interface is supposedly “native on the Twitter client”, so you might not realize that anyone was involved except Twitter — until you get your receipt.

Lavingia added that creating a purchase experience that’s “really simple and really fast has always been super important for us” and that he hopes that similar experiences will “over time become the norm because of things like Twitter.”

For its part, Twitter describes this as “an early step in our building functionality into Twitter to make shopping from mobile devices convenient and easy, hopefully even fun,” so you can probably expect its commerce initiatives to expand. (We’ve also heard that the company was working on something closer to a marketplace separate from the main river of content.)

This seems like a pretty natural avenue for expansion — not only is it a potential source of revenue on its own, but if Twitter successfully drives sales, that could encourage businesses to become more active on the service and to advertise more.